Exporting to Collada: Physics don't get exported
Assuming you want the motion generated by the rigid-body simulation (not the mass and other parameters), you can bake the simulation to keyframes.

There are two ways to do this:

Method one:
In the Tool shelf (N in the 3D view) > Rigid body tools, press Bake to Keyframes:



Method two:
Press 3D view > Header > Object > Animation > Bake action: